4. The Form Sense II 2021.pptx

The Form Sense II Introduction. Objectives and Readings. Factors affecting spatial resolution and visual acuity. SFO1004 Dr Sarah J Waugh© Objectives and readings The student should be able to: • Understand the change in visual acuity as a function of optical defocus (i.e. the impact of refracti...

The Form Sense II Introduction. Objectives and Readings. Factors affecting spatial resolution and visual acuity. SFO1004 Dr Sarah J Waugh© Objectives and readings The student should be able to: • Understand the change in visual acuity as a function of optical defocus (i.e. the impact of refractive errors) • Describe the effects of some of the important factors affecting visual acuity relevant to optometry. SFO1004 Optometry: Science, Techniques and clinical management. Rosenfield and Logan. • Chapter 12. https://webvision.med.utah.e du/ • Part VIII: Visual acuity Dr Sarah J Waugh© Factors that affect visual acuity • Many different factors may impact on visual acuity measurements. • Not surprising given that optimal visual acuity is a combination of many different factors and any one of them may contribute to a reduction. • We will consider some of the more important and relevant factors to optometrists. SFO1004 • • • • • • • • Refractive error Retinal location (eccentricity) Contrast Contour interactions (crowding) Luminance Meridional variations Movement of the eyes or target Developmental aspects Dr Sarah J Waugh© VA and refractive error • As soon as the optics of the eye become defocussed, the retinal image is also defocused which can result in blurry vision. • Emmetropia occurs when the optical quality is not diminished and the focus is conjugate with the retina. • Therefore, emmetropia does not impact on VA. SFO1004 https://webvision.med.utah.edu/book/part-viiipsychophysics-of-vision/visual-acuity/ Dr Sarah J Waugh© VA and refractive error • As soon as the optics of the eye become defocussed, the retinal image is also defocused which can result in blurry vision. • Myopia (short-sightedness) – one of the main refractive error types causes the retinal image to spread. SFO1004 https://webvision.med.utah.edu/book/part-viiipsychophysics-of-vision/visual-acuity/ Dr Sarah J Waugh© VA and refractive error • As soon as the optics of the eye become defocussed, the retinal image is also defocused which can result in blurry vision. • Hyperopia or hypermetropia (long sightedness) – one of the main refractive error types causes the retinal image to spread. SFO1004 https://webvision.med.utah.edu/book/part-viiipsychophysics-of-vision/visual-acuity/ Dr Sarah J Waugh© VA as a function of optical blur • Open squares (photopic background) – show that as the blurring lens increases in power, VA decreases. • Also shown are results using different levels of background luminance. What is the effect on VA as the luminance decreases? • Data from: Johnson and Casson. Effects of luminance, contrast and blur on visual acuity. Optometry & SFO1004 20/20 Dr Sarah J Waugh© • VA is best at the fovea (00 eccentricity). • The centre of the fovea is where visual acuity is best, which is why our eyes are designed to bring objects into focus at the fovea. • Even small deviations away from the fovea cause a large fall off in VA. • For patients that suffer from ocular diseases that affect the fovea (e.g. macular degeneration), this is an important consideration because central vision is lost. • SFO1004 Shown are data from a study by 6/6 Decimal notation VA and retinal eccentricity (or location) 6/60 https://webvision.med.utah.edu/ book/part-viii-psychophysics-ofvision/visual-acuity/ Dr Sarah J Waugh© VA and retinal eccentricity (or location) • It is possible to demonstrate the impact of retinal location on VA. • Anstis (1974) devised a vision chart that represents equal letter readability at different retinal locations. • The chart also shows how VA changes across the field of vision. • Anstis, S. A chart demonstrating variation in SFO1004 Dr Sarah J Waugh© VA and contrast • Contrast is defined as the ratio of the increment (or decrement in the case shown) of light luminance divided by the background luminance and referred to as the ‘Weber’ fraction ΔL/L. – Luminance of the background is measured (L). – Luminance of the letter is measured (ΔL). – Contrast (Weber) is specified as the ratio of ΔL/L. – For ‘black’ on ‘white’ it is sometimes SFO1004 designated. as a minus (-) value e.g. ETDRS high contrast chart Dr Sarah J Waugh© VA and contrast ETDRS low contrast charts – 2 different contrast levels • Visual acuity also reduces as the letter contrast reduces. • A variety of vision charts have been produced to measure low contrast VA. • Some follow the ETDRS format as shown, where the letters on a particular chart are all a fixed contrast and letter size varies. SFO1004 Dr Sarah J Waugh© VA and contrast • Other low contrast letter charts have the same size of letter, but contrast varies from high to low, such as the Pelli-Robson chart shown. • Low contrast VA charts can be helpful in assessing patients with low vision (e.g. cataract) and other conditions where normal high contrast charts may show relatively little defect. SFO1004 Image of Pelli-Robson low contrast chart Dr Sarah J Waugh© VA and contrast • Open squares (photopic background) show that as the contrast of the letters increases, VA also improves. • Also shown are results using different levels of background luminance. 20/20 High contrast level • Data from: Johnson and Casson. Effects of luminance, contrast and blur on visual acuity. Optometry & Vision Science 1995; 72: 864. SFO1004 Dr Sarah J Waugh© VA and the crowding effect • VA measured with single letters is usually avoided – unless the letters are surrounded by flankers. • The flankers could be bars or letters as shown in the images. • The impact of flankers on VA measurements is called crowding. • VA decreases as a function of the distance of the flankers to the target letter. • At the fovea crowding occurs over small distances up to about 5 min of arc. •SFO1004 Crowding is stronger in the periphery. Dr Sarah J Waugh© VA and the crowding effect • Measuring visual acuity using crowded charts is thought to be the best. • The logMAR form of VA charts are crowded and are preferred particularly when accurate VA measurements are required. • The spacing between letters and rows is proportional to the size and is equal. SFO1004 Dr Sarah J Waugh© Fixate on the cross – notice that the unflanked ‘H’ is easiest to see? Fixate on the cross – notice that the unflanked ‘H’ is easiest to see? VA and luminance • Data showing visual acuity as a function of background luminance. • Notice the ‘rod-cone’ break as the luminance decreases to scotopic levels. • Notice that visual acuity remains good for a wide range of photopic luminances. https://webvision.med.utah.edu/book/p art-viii-psychophysics-of-vision/visual-a cuity/ SFO1004 Dr Sarah J Waugh© Luminance – clinical measurements under mesopic conditions • Under mesopic light levels, visual acuity is reduced. • Data from a study where visual acuity was measured for photopic (228 cd/m2) and mesopic (0.164 cd/m2) light levels. • Resulted in a decrease in visual acuity on average of 0.48 logMAR (almost 5 lines on a standard ETDRS chart). • VA measurements under mesopic conditions have not SFO1004 been standardised. Pluhacek F. & Siderov J. Mesopic visual acuity is less crowded. Graefe’s Archive for Clinical and Experimental Ophthalmology, 2018; 256:1739–1746. Dr Sarah J Waugh© VA and luminance – chart luminance • Most clinical measurements of visual acuity are made under photopic luminances. • General room lighting can often be subdued (i.e. not fully bright). • Variations in chart luminance occur due to the different displays that is used. • In general chart luminances should be somewhere between 80-320 cd/m2. • It is unlikely that visual acuity will vary much within this range. – Projectors, computerised chart displays, internally illuminated charts SFO1004 Dr Sarah J Waugh© Orientation – oblique effect • The resolving power of the eye appears to be less for oblique orientations. • Measurements require the use of a grating type pattern although the measured effects may not be very large (~15% differences). • One of the reasons that use of Landolt C at oblique angles are avoided. SFO1004 Dr Sarah J Waugh© Movement of the eyes or target • Even under steady fixation, the eyes are in constant motion. • Under normal conditions, these small (fixational) eye movements do not detract from good visual acuity. • For larger eye movements or target velocity (greater than about 3 degrees/second, there is a decrease in visual acuity. • Data from: Westheimer and McKee. Visual acuity in the presence of retinal image motion. J Optical SFO1004 Society of America, 1975;65:847- Dr Sarah J Waugh© VA and developmental aspects • Visual acuity varies with age. • Following birth visual acuity gradually improves as the visual structures mature and reaches adult like values at around 5-6 years of age. • With age visual acuity gradually declines (over about age 60), probably due to the increase in light scatter within the eye. • Data from: Sonksen PM, Wade AM, Proffitt R, Heavens S, Salt AT. The Sonksen logMAR test of visual acuity: II. Age norms from 2 years 9 months to 8 years. Journal of American Association for Pediatric Ophthalmology and SFO1004 Strabismus. 2008 Feb 1;12(1):18-22. Dr Sarah J Waugh© Questions? • Remember you can ask questions via the module Teams site. SFO1004 Dr Sarah J Waugh©